Amplifier stages are a key part of many electronic systems. A generic amplifier stage allows amplifying (even just by regenerating it) an electrical signal being input thereto. In this way, subsequent processing of the signal performed by additional components of the electronic system downstream form the amplifier stage is simplified. Accordingly, the design requirements of the downstream components of the amplifier stage are less stringent.
An example application of the amplifier stages is the generation of control signals in non-volatile memories, such as FLASH-type memories.
FLASH memories have a wide diffusion in various types of electronic devices (from smart cards to data storage systems). A generic FLASH memory comprises a plurality of memory cells arranged in a matrix having a plurality of rows and columns. As it is known, each memory cell comprises a floating gate transistor, which is used to store information (for example, one bit) on the basis of a quantity of charge carriers (such as electrons or holes) trapped in its gate which is electrically floating.
The charge carriers are injected into and extracted from the floating gate by stimulating physical phenomena such as Fowler-Nordheim (FN) tunneling or Hot Carrier Injection (HCI, such as Channel Hot Electron—CHE). In order to stimulate such a physical phenomena, high bias voltages to be applied to the memory cells (e.g., on the order of tens of volts) are generally required.
In addition, it is also necessary to be able to vary such bias voltages fast so as to perform sequences of erase operations, program and verify operations, and read operations in the memory cells of the FLASH memory.
The generation of the bias voltages is performed by a pulse generator element within a read/write unit of the FLASH memory selectively connected to each memory cell by corresponding signal lines (such as word lines and bit lines). This pulse generator element comprises a Digital-to-Analog Converter (DAC) for generating an (analog low-voltage) version of each bias voltage. The analog low-voltage is supplied to an amplifier stage that amplifies the bias voltage into a range of values comprising those required to operate on the selected memory cell (or on more selected memory cells simultaneously).
The amplifier stage generally comprises a pre-amplifier transconductance module (e.g., a Transconductance Operational Amplifier—OTA) that receives the bias voltage from the DAC, and an output amplification module that provides the amplified bias voltage. Generally, the (transconductance) pre-amplifier module and the output (amplification) module are connected in such a way to form a feedback loop.
The performance of the amplifier stage (e.g., in terms of speed for the same power consumption) is heavily affected by a capacitive load connected at its output. However, in the specific case of the non-volatile memories, this capacitive load is extremely variable (e.g., in a range on the order of hundreds of pF) according to the state of the selected memory cells. Therefore, also the performance of the amplifier stage is not uniform between different operations since the charging and discharging times of the capacitive load vary according to the capacitive load. It may generate significant over- and under-elongations compared to the desired values of the bias voltage.
In the art, inside the amplifier stage discharge circuits are implemented for speeding up a discharge operation of the capacitive load, so as to increase the performance in terms of speed. Unfortunately, such a device involves a substantial asymmetry between the charging and discharging phases of the capacitive load.
Furthermore, in order to obtain values of the bias voltage higher than a power supply voltage of the FLASH memory (e.g., necessary to stimulate the FN tunneling) the output amplifying element is supplied via a voltage multiplier circuit, such as a charge pump. The charge pump uses capacitive elements to provide voltage values higher than the value of the supply voltage, so that the power that may be supplied by the charge pump is limited (as depending on the electrical charge accumulated in these capacitive elements).
Unfortunately, direct electrical currents of the output module (e.g., a bias current and a leakage current) and of the discharge circuits (electrically coupled with the charge pump) may significantly increase the static power consumption of the amplifier stage (reducing the performance from a power consumption point of view).